Recording exhaust sounds

[Stryker6040]

I want to record some exhaust sounds and was wondering where the best place would be to place the iPhone. I know they are the the best but would like to get a before and after of the borla attack system. Where should I place the iPhone and how far back?

 

[Fatherof3]

Either on the ground right where the trailer hitch would be or maybe on the bumper with the licence plate ?